1. Field of the Invention
The invention relates generally to methods and systems involving cutting tools in oilfield applications.
2. Background Art
FIG. 1 shows one example of a conventional drilling system for drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drilling tool assembly 12 that extends downward into a well bore 14. The drilling tool assembly 12 includes a drilling string 16, and a bottomhole assembly (BHA) 18, which is attached to the distal end of the drill string 16. The “distal end” of the drill string is the end furthest from the drilling rig.
The drill string 16 includes several joints of drill pipe 16a connected end to end through tool joints 16b. The drill string 16 is used to transmit drilling fluid (through its hollow core) and to transmit rotational power from the drill rig 10 to the BHA 18. In some cases the drill string 16 further includes additional components such as subs, pup joints, etc.
The BHA 18 includes at least a drill bit 20. Typical BHA's may also include additional components attached between the drill string 16 and the drill bit 20. Examples of additional BHA components include drill collars, stabilizers, measurement-while-drilling (MWD) tools, logging-while-drilling (LWD) tools, subs, hole enlargement devices (e.g., hole openers and reamers), jars, accelerators, thrusters, downhole motors, and rotary steerable systems. In certain BHA designs, the BHA may include either a drill bit 20 or at least one secondary cutting structure.
In general, drilling tool assemblies 12 may include other drilling components and accessories, such as special valves, such as kelly cocks, blowout preventers, and safety valves. Additional components included in a drilling tool assembly 12 may be considered a part of the drill string 16 or a part of the BHA 18 depending on their locations in the drilling tool assembly 12.
The drill bit 20 in the BHA 18 may be any type of drill bit suitable for drilling earth formation. Two common types of drill bits used for drilling earth formations are fixed-cutter (or fixed-head) bits and roller cone bits. FIG. 2 shows one example of a fixed-cutter bit. FIG. 3 shows one example of a roller cone bit.
Referring to FIG. 2, fixed-cutter bits (also called drag bits) 21 typically comprise a bit body 22 having a threaded connection at one end 24 and a cutting head 26 formed at the other end. The head 26 of the fixed-cutter bit 21 typically includes a plurality of ribs or blades 28 arranged about the rotational axis of the drill bit and extending radially outward from the bit body 22. Cutting elements 29 are embedded in the raised ribs 28 to cut formation as the drill bit is rotated on a bottom surface of a well bore. Cutting elements 29 of fixed-cutter bits typically comprise polycrystalline diamond compacts (PDC) or specially manufactured diamond cutters. These drill bits are also referred to as PDC bits.
Referring to FIG. 3, roller cone bits 30 typically comprise a bit body 32 having a threaded connection at one end 34 and one or more legs (typically three) extending from the other end. A roller cone 36 is mounted on each leg and is able to rotate with respect to the bit body 32. On each cone 36 of the drill bit 30 are a plurality of cutting elements 38, typically arranged in rows about the surface of the cone 36 to contact and cut through formation encountered by the drill bit. Roller cone bits 30 are designed such that as a drill bit rotates, the cones 36 of the roller cone bit 30 roll on the bottom surface of the well bore (called the “bottomhole”) and the cutting elements 38 scrape and crush the formation beneath them. In some cases, the cutting elements 38 on the roller cone bit 30 comprise milled steel teeth formed on the surface of the cones 36. In other cases, the cutting elements 38 comprise inserts embedded in the cones. Typically, these inserts are tungsten carbide inserts or polycrystalline diamond compacts. In some cases hardfacing is applied to the surface of the cutting elements and/or cones to improve wear resistance of the cutting structure.
For a drill bit 20 to drill through formation, sufficient rotational moment and axial force must be applied to the drill bit 20 to cause the cutting elements of the drill bit 20 to cut into and/or crush formation as the drill bit is rotated. The axial force applied on the drill bit 20 is typically referred to as the “weight on bit” (WOB). The rotational moment applied to the drilling tool assembly 12 at the drill rig 10 (usually by a rotary table or a top drive mechanism) to turn the drilling tool assembly 12 is referred to as the “rotary torque”. The speed at which the rotary table rotates the drilling tool assembly 12, typically measured in revolutions per minute (RPM), is referred to as the “rotary speed”. Additionally, the portion of the weight of the drilling tool assembly supported at the rig 10 by the suspending mechanism (or hook) is typically referred to as the hook load.
As the drilling industry continues to evolve, methods of simulating and/or modeling the performance of components used in the drilling industry have begun to be developed. Drilling tool assemblies can extend more than a mile in length while being less than a foot in diameter. As a result, these assemblies are relatively flexible along their length and may vibrate when driven rotationally by the rotary table. Drilling tool assembly vibrations may also result from vibration of the drill bit during drilling. Several modes of vibration are possible for drilling tool assemblies. In general, drilling tool assemblies may experience torsional, axial, and lateral vibrations. Although partial damping of vibration may result due to viscosity of drilling fluid, friction of the drill pipe rubbing against the wall of the well bore, energy absorbed in drilling the formation, and drilling tool assembly impacting with well bore wall, these sources of damping are typically not enough to suppress vibrations completely.
One example of a method that may be used to simulate a drilling tool assembly is disclosed in U.S. patent application Ser. No. 09/689,299 entitled “Simulating the Dynamic Response of a Drilling Tool Assembly and its Application to Drilling Tool Assembly Design Optimizing and Drilling Performance Optimization”, which is incorporated by reference in its entirety.
Vibrations of a drilling tool assembly are difficult to predict because different forces may combine to produce the various modes of vibration, and models for simulating the response of an entire drilling tool assembly including a drill bit interacting with formation in a drilling environment have not been available. Drilling tool assembly vibrations are generally undesirable, not only because they are difficult to predict, but also because the vibrations can significantly affect the instantaneous force applied on the drill bit. This can result in the drill bit not operating as expected.
For example, vibrations can result in off-centered drilling, slower rates of penetration, excessive wear of the cutting elements, or premature failure of the cutting elements and the drill bit. Lateral vibration of the drilling tool assembly may be a result of radial force imbalances, mass imbalance, and drill bit/formation interaction, among other things. Lateral vibration results in poor drilling tool assembly performance, overgage hole drilling, out-of-round) or “lobed” well bores and premature failure of both the cutting elements and drill bit bearings. Lateral vibration is particularly problematic if hole openers are used.
During drilling operations, it may be desirable to increase the diameter of the drilled wellbore to a selected larger diameter. Further, increasing the diameter of the wellbore may be necessary if, for example, the formation being drilled is unstable such that the wellbore diameter changes after being drilled by the drill bit. Accordingly, tools known in the art such as “hole openers” and “underreamers” have been used to enlarge diameters of drilled wellbores.
In some drilling environments, it may be advantageous, from an ease of drilling standpoint, to drill a smaller diameter borehole (e.g., an 8½ inch diameter hole) before opening or underreaming the borehole to a larger diameter (e.g., to a 17½ inch diameter hole). Other circumstances in which first drilling smaller hole and then underreaming or opening the hole include directionally drilled boreholes. It is difficult to directionally drill a wellbore with a large diameter bit because, for example, larger diameter bits have an increased tendency to “torque-up” (or stick) in the wellbore. When a larger diameter bit “torques-up”, the bit tends to drill a tortuous trajectory because it periodically sticks and then frees up and unloads torque. Therefore it is often advantageous to directionally drill a smaller diameter hole before running a hole opener in the wellbore to increase the wellbore to a desired larger diameter.
A typical prior art hole opener is disclosed in U.S. Pat. No. 4,630,694 issued to Walton et al. The hole opener disclosed in the '694 patent includes a bull nose, a pilot section, and an elongated body adapted to be connected to a drillstring used to drill a wellbore. The hole opener also includes a triangularly arranged, hardfaced blade structure adapted to increase a diameter of the wellbore.
Another prior art hole opener is disclosed in U.S. Pat. No. 5,035,293 issued to Rives. The hole opener disclosed in the '293 patent may be used either as a sub in a drill string, or may be coupled to the bottom end of a drill string in a manner similar to a drill bit. This particular hole opener includes radially spaced blades with cutting elements and shock absorbers disposed thereon.
Other prior art hole openers include, for example, rotatable cutters affixed to a tool body in a cantilever fashion. Such a hole opener is shown, for example, in U.S. Pat. No. 5,992,542 issued to Rives. The hole opener disclosed in the '542 patent includes hardfaced cutter shells that are similar to roller cones used with roller cone drill bits.
U.S. Patent Publication No. 2004/0222025, which is assigned to the assignee of the present invention, and is incorporated by reference in its entirety, discloses a hole opener wherein cutting elements may be positioned on the respective blades so as to balance a force or work distribution and provide a force or work balanced cutting structure. “Force balance” may refer to a substantial balancing of any force during drilling (lateral, axial, torsional, and/or vibrational, for example). One method of later force balancing has been described in detail in, for example, T. M. Warren et al., Drag Bit Performance Modeling, paper no. 15617, Society of Petroleum Engineers, Richardson, Tex., 1986. Similarly, “work balance” refers to a substantial balancing of work performed between the blades and between cutting elements on the blades.
The term “work” used in that publication is defined as follows. A cutting element on the blades during drilling operations cuts the earth formation through a combination of axial penetration and lateral scraping. The movement of the cutting element through the formation can thus be separated into a “lateral scraping” component and an “axial crushing” component. The distance that the cutting element moves laterally, that is, in the plane of the bottom of the wellbore, is called the lateral displacement. The distance that the cutting element moves in the axial direction is called the vertical displacement. The force vector acting on the cutting element can also be characterized by a lateral force component acting in the plane of the bottom of the wellbore and a vertical force component acting along the axis of the drill bit. The work done by a cutting element is defined as the product of the force required to move the cutting element and the displacement of the cutting element in the direction of the force.
Thus, the lateral work done by the cutting element is the product of the lateral force and the lateral displacement. Similarly, the vertical (axial) work done is the product of the vertical force and the vertical displacement. The total work done by each cutting element can be calculated by summing the vertical work and the lateral work. Summing the total work done by each cutting element on any one blade will provide the total work done by that blade.
Force balancing and work balancing may also refer to a substantial balancing of forces and work between corresponding cutting elements, between redundant cutting elements, etc. Balancing may also be performed over the entire hole opener (e.g., over the entire cutting structure).
What is still needed, however, are methods for coupling the behavior of drill bits, hole openers, and other tools to one another in order to optimize the drilling performance of a BHA assembly.